Some heat exchangers include flattened tubes that extend between and are supported by two spaced apart header plates. Each header plate typically includes openings that are shaped and sized to receive ends of the flattened tubes. A fluid tank is fitted and sealed to each header plate to receive heat exchange fluid that flows through the flattened tubes. Such heat exchangers commonly experience thermal cycle stress due to fluids of different temperatures passing through the flattened tubes. In particular, different temperature fluids may cause expansion and/or contraction of the flattened tubes, creating stresses at joints between the tubes and the header plates. As a result, the joints, or the flattened tubes themselves, may fracture, resulting in leakage and possible failure of the heat exchanger.